。
She was dressed entirely in black.
Literal
She [topic-は] black-completely 's outfit [object-を] was-doing.
~ずくめ is a productive suffix meaning 'all in, nothing but, entirely' — attached to nouns or noun-like elements. 黒ずくめ ('all in black') is one of its most common forms, often evoking mystery, mourning, or sleek style. Other examples: いいことずくめ ('nothing but good things'), 規則ずくめ ('rules upon rules'). 服装をする ('to dress, to be dressed') is a noun + light verb pairing, parallel to 顔をする ('have a face') and スタイルをしている ('have a figure'). The past 服装をしていた fixes the dress in a remembered scene.